Glass fiber reinforced plastics have been used widely in recent years in the automotive industry. These plastics include sheet molding compound, glass fiber reinforced reaction injection moldable materials and various other glass fiber reinforced plastics. The advantages of lightweight, high strength, rustproofing and relatively low cost make them ideal for many automotive exterior body panel applications.
Of these materials, the reinforced reaction injection moldable material (RRIM) is of particular interest to the automotive industry since they can be processed economically with low cost equipment. RRIM materials filled with milled glass have greatly improved stiffness and are suitable for applications where structural integrity is required. These materials are also suitable for use in large automobile exterior body panels since they produce readily paintable surfaces. However, one inherent drawback of these RRIM materials is that they have a relatively high coefficient of thermal expansion, i.e., 33.times.10.sup.-6 /.degree.C. in the direction parallel to flow, 108.times.10.sup.-6 /.degree.C. in the direction perpendicular to flow, when compared to that of only 12.times.10.sup.-6 /.degree.C. for steel. Consequently, when used in relatively large and flat panels where the service life of the part covers a wide temperature span (e.g., -20.degree. C. to +70.degree. C.), the dimensional stability of the RRIM materials is often less than desirable.
To meet the requirement of dimensional stability on a large body panel, a new type of glass fiber reinforced RIM material was subsequently developed by using a glass fiber mat of continuous glass. The presence of a glass fiber mat embedded in a large RIM part through the whole area dramatically improves its dimensional stability. This is achieved while all the other desirable mechanical and processing properties are maintained.
The processing of glass fiber mat reinforced RIM is relatively simple. It involves placing a glass fiber mat in a mold cavity and shooting RIM material into the closed mold so that the RIM material is soaked through the mat. A completed part is then removed after it is cured in the mold.
A typical RIM used in this process is a polyurethane based material produced from two components: an isocyanate and a polyol. The in-mold pressure normally seen in the RIM process is less than 50 psi, requiring much less clamping force than other processes.
The glass mat reinforced RIM material is especially suitable for large body panels such as door panels or quarter panels on a vehicle. The dimensional stability of a resulting door panel or quarter panel is greatly improved while the traditional characteristics of RRIM, i.e., stiffness, strength, and ease of processing are maintained. The coefficient of thermal expansion of a glass mat reinforced RIM material is only one-third (13.times.10.sup.-6 /.degree.C.) of that for a RRIM material filled with milled glass. As a matter of fact, its thermal stability property is even superior to that of aluminum.
In the processing of glass fiber mat reinforced RIM parts where aesthetic property is important, however, a new problem has arisen. This is the glass fiber readout problem observed in the surface layer of a glass mat reinforced RIM part. The readout problem is caused largely by the presence of the continuous glass fiber in the surface layer of the panel. When a panel is situated in a mold under compression, the resin material located between the panel surface and a glass fiber in the surface layer of the panel is under higher pressure than the resin material located not adjacent to a glass fiber. As a consequence, when the part is demolded, the cured viscoelastic resin material located adjacent to a glass fiber will expand more than the resin material not adjacent to a glass fiber. This results in a panel with a surface showing protruded contours of various glass fibers which are located immediately below the surface of the panel, commonly known as the glass fiber readout problem.
Numerous efforts have been made to correct this glass fiber readout problem. However, none of them was found to work satisfactory in hiding the glass readout on the surface of a RIM part. For instance, an in-mold coating process used on a conventional RIM part disclosed in U.S. Pat. No. 4,282,285 was found inadequate to correct the problem. This fiber readout problem makes the glass fiber mat reinforced RIM material unsuitable for exterior automobile body panel applications for aesthetic reasons.
It is therefore an object of the present invention to provide a method of producing glass mat reinforced RIM panels with smooth surfaces without the fiber readout problem.
It is yet another object of the present invention to provide a method of making glass mat reinforced RIM panels for automotive exterior body panel applications having smooth surfaces adequate to obtain a class-A painted surface.
It is a further objective of the present invention to provide a method of producing glass mat reinforced RIM panels for automotive exterior body panel applications having smooth and predecorated surfaces that do not require further painting or decorating.